1. Field of the Invention
The present invention relates to a hermetic electric compressor for use, particularly, in a car air conditioner.
2. Description of the Prior Art
FIGS. 8 and 9 show a conventional hermetic electric compressor. In FIG. 8, the compressor as represented by numeral 12 includes a sealed casing 12A. The sealed casing 12A includes therein a three-phase motor unit 1, and a compressor unit 6 driven by the motor unit 1 for compressing refrigerant gas. The compressor 12 further includes a discharge pipe 4 for discharging the compressed refrigerant gas outside the sealed casing 12A for a refrigerating cycle. As shown in FIG. 9, the motor unit 1 includes a stator having a coil unit 2 with three coils. The three coils are connected to an inverter unit 21 via three-phase terminals U, V and W, respectively, for receiving alternating current of a controlled frequency. A thermostat 3 is firmly tied at a proper portion on the coil unit 2 or between the coils, using proper strings for this purpose, and is connected to a control circuit 20 of the inverter unit 21.
With this arrangement, when the compressor 12 is overloaded so that a temperature of the coil unit 2 increases to exceed a preset value, the thermostat 3 is operated to open the circuit. The control circuit 20 detects it and stops energization to the coil unit 2 for preventing damage of the coil unit 2 due to heat.
In another conventional hermetic electric compressor, a thermistor 5 is attached to the discharge pipe 4, instead of the thermostat 3 at the coil unit 2 in the foregoing compressor, for monitoring temperatures of the discharged gas at the discharge pipe 4, which is also shown in FIG. 8. As also shown in FIG. 9, the thermistor 5 is connected to the control circuit 20. With this arrangement, when the monitored temperature exceeds a preset value, the control circuit 20 detects it to stop energization to the coil unit 2, or alternatively, the control unit 20 detects it to lower a frequency of the alternating current fed to the coil unit 2, that is, a rotational frequency or speed of a rotor of the motor unit 19 to a preset value for preventing damage of the coil unit 2 due to heat.
However, in the foregoing conventional compressors, there have been the following problems:
When energization to the coil unit 2 is stopped by operating the thermostat 3, several minutes are necessary for the thermostat 3 to be restored to restart the operation of the compressor. Thus, if the compressor is applied to the car air conditioner, since the car air conditioner is stopped in operation for ten and several minutes, the inner surfaces of window glasses of a car may be clouded up depending on conditions of the inside air and the outside air. This may raise a serious problem to the car driving. Further, the air condition inside the car may be extremely deteriorated.
On the other hand, when energization to the coil unit 2 is stopped or a frequency of the alternating current fed to the coil unit 2 is lowered to reduce the load of the compressor by using the thermistor 5, the following problem may be encountered: During a normal operation of the compressor, the coil unit 2 is cooled by the compressed gas so that a difference in temperature between the discharged gas and the coil unit 2 is held at 5.about.10 degrees. On the other hand, during an overload operation of the compressor or when a compression ratio is large, a gas circulation amount is extremely reduced, and thus, the cooling of the coil unit 2 by means of the compressed gas becomes insufficient. In this case, it possible that a difference in temperature between the discharged gas and the coil unit 2 becomes more than 20 degrees. Further, since the ambient temperature around the discharge pipe 4 affects the temperature of the discharged gas at the discharge pipe 4, when the ambient temperature is quite low, a difference between the temperature of the discharged gas as monitored by the thermistor 5 and the coil temperature increases. As a result, the compressor continues to be operated even when the coil temperature actually exceeds the preset value so that the coil unit 2 is seriously damaged due to heat.
Further, since the thermostat 3 and the control circuit 20 in the former compressor and the thermistor 5 and the control circuit 20 in the latter compressor are connected via general leads, that is, general insulated wires, as shown in FIG. 8, it is possible that electrical noise caused by operation of the inverter unit 21 or the like may enter a signal line to the control circuit 20 to cause malfunction thereof.